From: Brian Gr (brian_gr_at_hotmail.com)
Date: 2001-12-25 17:04:03
EEG People,
I still see tremendous problems in making soundblaster work in this application. (See my post: http://groups.yahoo.com/group/buildcheapeeg/message/545for reasons one can't get more than 4 bit resolution at EEG sampling frequencies with a soundblaster in FM modulation.)
AM is not much better. When we get to actual details of trying to implementthis solution, one would be sampling 16 bit data stream at 44KHz (about 10Mbyte/minute of data to process for ONE channel) The solution is not scalable...how would 2 or more channels be implemented? Multiple sound cards? Multiple carrier frequencies? That would be interesting to decode in software on the PC. In comparison to the modular EEG method (10 bit AD converter with 6 channels) this soundblaster method is a path of much higher resistance.Each additional channel imposes huge extra processor requirements at the PC end. The hardware to implement this will have poor resolution (about 4bits resolution using FM, certainly not 16 bit resolution using AM) Whereas, the modularEEG is a very straightforward from hardware and software perspective, not complex, lo processor overhead, proven accuracy, much easier to isolate, works with any PC, etc.
I have a laptop (Toshiba Tecra 8100) that doesn't have a line in available without purchasing an expensive docking station. But, it has a serial port,with basically the same configuration as every other PC. What would we dowith anyone who doesn't have full duplex sound card. These cards can't have both audio in and out simultaneously. So, audio biofeedback would requirepurchase of an additional card.
I suggest we put the soundblaster approach aside and use a standard, inexpensive approach as has been designed in the modular_EEG design.
Seasons greetings! Best wishes for the new year.
Brian.
----- Original Message -----
From: Andreas Robinson
To: buildcheapeeg_at_yahoogroups.com
Sent: Tuesday, December 25, 2001 6:39 AM
Subject: [buildcheapeeg] Amplitude modulation
Hello all, and merry christmas!
I did some calculating on AM transmission about a
month ago...
The conclusion is, you can do it, but it is not easy
and probably not cheap either.
The idea is like this, moving from the frontend to the
sound card:
1. First you have a regular input stage, amplifying
the signal about 150 times.
2. The signal is filtered so that you get 96 dB
attenuation at 9kHz. A third order Chebyshev I filter
is enough.
3. The signal is passed through an analog optocoupler
(HCNR200 or IL300)
4. The signal is amplitude modulated to 10kHz. This is
the tricky part. If it is done badly, a lot of
resolution is lost.
There are two ways to do this:
* Modulate using a switch, flipping it back and forth
between the signal and a constant voltage at a rate of
10kHz. Then filter the signal using a 6th order
elliptic filter. If you don't do that, you'll get
aliasing in the sound card.
or
* Modulate with a sine wave, the old fashioned way.
Requires an experienced analog designer to get it
right.
5. A buffer amplifier sends the signal along to the
sound card.
6. The sound card samples the signal in 40kHz or
higher.
Strengths: 16-bit resolution possible, at least in
theory.
Weaknesses: More complex analog section. It requires
either a very high filter order (total of 9) or a low
distortion sine-wave oscillator and modulator. It also
needs more power because it uses one analog
opto-coupler per channel. You have to use two low
noise power supplies (such as batteries) as well.
/Andreas
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST